1. Field of the Invention
This invention relates to a cylinder-by-cylinder air-fuel ratio-estimating system for internal combustion engines, which estimates the air-fuel ratio of a mixture supplied to each of cylinders of the engine by applying an observer based on a modern control theory.
2. Prior Art
Conventionally, there is known a cylinder-by-cylinder air-fuel ratio-estimating system for internal combustion engines, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 6-173755, which sets an observer for observing the internal operative state of the exhaust system of the engine, based on a model describing the behavior of the exhaust system, and estimates the air-fuel ratio of a mixture supplied to each of cylinders of the engine (cylinder-by-cylinder air-fuel ratio), based on an output from an air-fuel ratio sensor arranged in the exhaust system at a confluent portion thereof, for generating an output proportional to the air-fuel ratio.
According to the known cylinder-by-cylinder air-fuel ratio-estimating system, in view of the fact that a large change occurs in the cylinder-by-cylinder air-fuel ratio estimated by the observer, e.g. when the internal combustion engine is in a transient operating condition, the estimated cylinder-by-cylinder air-fuel ratio is initialized to a value corresponding to a stoichiometric air-fuel ratio when the estimated air-fuel ratio falls outside a range between predetermined upper and lower limit values.
According to the above manner of initialization of the cylinder-by-cylinder air-fuel ratio, the estimated cylinder-by-cylinder air-fuel ratio is initialized to a value corresponding to the stoichiometric air-fuel ratio when the engine is in a transient operating condition, such as when the cylinder-by-cylinder air-fuel ratio feedback control is started, or when the engine is in a condition immediately after a fuel-cut state (the fuel supply has just been resumed). This, however, can cause a delay in the convergence of the observer output (estimated cylinder-by-cylinder air-fuel ratio). As a result, the actual cylinder-by-cylinder air-fuel ratio, which is obtained based on results of the air-fuel ratio feedback control by the use of the observer output, can deviate from a desired air-fuel ratio.